TV Tuner cards (on topic, honest :)

From: Richard Erlacher <edick_at_idcomm.com>
Date: Thu Dec 14 15:32:29 2000

The easiest way to deal with it is to hook a 'scope to the vertical sync
line, triggering on it and see if you get a stabile sync on the second
channel hooked to horizontal sync. if it does look stabile, it's not
interlacing. with interlace (at 30 Hz) every other field is off by half a
horizontal period. It should be pretty apparent, since the display is
difficult to view at that low frequency anyway. If the relation between
vertical-sync and horizontal sync is rock-solid, as I recall it was on most
video displays of the '80's, it's because they didn't interlace.

If you've only got a single trace 'scope, then you just have to look a the
period of the vsync with your trigger set to the vertical sync and
displaying two such pulses. The period will not be precisely half what it
is for two cycles as defined in Hsync periods. The period should be 262.5
hsync periods if it's interlacing. Normally it was on the order of 512 on
the typical video display circuit, only because it was easy to generate. In
some cases it's as long as 640 or even 768, with a long blanking window. If
you trigger on hsync and can't get a stabile display of the vsync it's
probably interlaced. If you trigger on vsync and can't get a stabile
display of hsync, then, likewise, it's because the hsync is half-a-cycle off
the verical rate, which means they're interlacing.

The easiest way is to look at a schematic. Also, if the display is
something line 16 lines of 64 characters, e.g. TRS-80, or Digital Group,
then it's likely it's not interlaced. If it's something like a 640 (or 512)
x 384 (or 512) raster, it's likely to be interlaced. The bandwidth of most
monitors from the Ohio Scientific days interlaced only when there were large
numbers off pixels. Those couldn't be displayed on low-cost displays
without interlace because of the requirement for high ( > 10 MHz) bandwidth
from the video amps. Remember that a non-interlaced and non-flickering
display required a 60 Hz frame rate with the typical (P4?) phosphor of the
time. Interlace does flicker somewhat.

Dick

----- Original Message -----
From: "Bill Sudbrink" <bills_at_adrenaline.com>
To: <classiccmp_at_classiccmp.org>
Sent: Thursday, December 14, 2000 1:27 PM
Subject: RE: TV Tuner cards (on topic, honest :)


> > > One thing I have discovered... For whatever reason, the two TV cards
I
> > > have tried will not sync to Ohio Scientific boxes. I have several
> > > composite monitors that will, as well as two TV sets with composite
in.
> > > I also tried a Radio Shack RF modulator box with about the same
results.
> >
> > The problem may lie in that the Ohio Scientific boxes, though their
output
> > works fine on NTSC monitors, isn't interlaced. Do you suppose
> > that might be the problem?
>
> Hmmm... how would I check this?
>
>
Received on Thu Dec 14 2000 - 15:32:29 GMT